[D‐Ala2, Phe (p‐NO2)4, Leu5]Enkephalin Amide and Nα‐[D‐Ala2, Leu5]‐Enkephalyl‐Nε‐Acetyl‐Lysine Amide: Synthesis and Biological Properties of Prospective Enkephalin Cooperative‐Affinity and Photoaffinity Labels. Preliminary communication | Publicación